SSHPool in utils should allow customized host key missing policy

The cinder/utils SSHPool should allow  missing key policy and host key
file being customized so that any caller can determine by their own
scenario if the host key file can be customized, or if an 'AutoAdd' is
appropriate, or just reject the key when mismatch. This will give more
flexible customization and also prevent any security issue as a middle
man.

Closes-Bug: #1320056
Change-Id: I3c72b0d042de719ecd45429d376bd88d0aefb2cc
Merge "SSHPool in utils should allow customized host key missing policy"